Neural development requires the generation of a remarkable diversity of neural cell types and the stereotyped and often highly ordered specification of synaptic connections between these cells. A large number of studies have demonstrated that receptor-configured protein-tyrosine kinases (PTKs) play central roles in both of these events. Recent work from our laboratory and others has provided strong circumstantial evidence that several members of what is the largest sub-family of receptor PTKs in vertebrates-the Eph receptors-function to pattern topographic axonal projects that map ordered synaptic activity in one set of neurons onto another. Perhaps the best studied of such maps connects the retinal ganglion cells of the eye to their targets in the superior colliculus (SC) of the midbrain. In this map, a gradient of EphA receptors in the retina is hypothesized to interact with a complementary gradient of axon-repellant ephrin-A ligands in the SC. We propose to experimentally manipulate and analyze a batter of recently generated mouse lines that carry both loss-of-function and gain-of-functions mutations in the genes encoding many of these receptors and ligands, and to thereby assess in vivo the validity of previously advanced but largely untested hypotheses as to the mechanism of action of the EphA signaling system. We will also use these same mouse mutants to experimentally assess the functional relevance of this signaling system to the specificity of synaptic innervation from motor neurons in the spinal cord to their muscle targets in the limbs and the body wall.